124 



PHYSICAL BASIS OF HEREDITY 



between 47.7 and 79.3 is due to double crossovers as the 

 data for the intermediate regions show: 



star 



24.1 



79.3 



Dachs deficiency 



Black 



47.7 



Purple 



Vestigial 



Speck 



There was known to be present in this case a factor called 

 ^ ' deficiency ' ^ in one of the two second chromosomes 

 involved. It is near ' ' dachs ' ' and cuts down the crossing 

 over between star and speck by about 25.8 units. It 

 will be noticed that while the second summation value 

 for star speck, viz., 79.3, and the first value, viz., 104.4, are 

 very different, the crossover value between star and speck 

 is in one case 47.7 and in the other 48.7. The meaning of 

 this, as shown by data for intermediate loci, is that by the 

 addition of 25.1 units (104.4 minus 79.3 equals 25.1) the 

 number of double crossovers has so greatly increased that 

 a difference of only about 1 per cent, of apparent crossing 

 over is recorded in the star speck value. 



"Distance" and Linear Order 



The linear order of the genes implies distance between 

 them, for w^hich the crossover values stand as indices. It 

 is obvious that if the order of the genes remained the same 

 but something doubled the number of crossovers between 

 two loci, their ''distance'^ apart would at the same time 

 appear to have been doubled. Again, if crossing over is 

 thought of as due to twisting of the chromosomes of a pair 

 about each other, then if the twisting is more likely to occur 

 at the ends of the chromosomes, or if the twists themselves 



